Interface clock management

ABSTRACT

The timing of the synchronous interface is controlled by a clock signal driven by a controller. The clock is toggled in order to send a command to a memory device via the interface. If there are no additional commands to be sent via the interface, the controller suspends the clock signal. When the memory device is ready, the memory device drives a signal back to the controller. The timing of this signal is not dependent upon the clock signal. Receipt of this signal by the controller indicates that the memory device is ready and the clock signal should be resumed so that a status of the command can be returned via the interface, or another command issued via the interface.

TECHNICAL FIELD

The present disclosure relates to techniques for communicatinginformation between circuits. More specifically, but not exclusively,the present disclosure relates to communication protocols and signalsbetween a memory controller and a memory device.

BACKGROUND

Semiconductor memory is an important part of modern electronics.Semiconductor memory may be divided into major categories. Two of thesecategories include volatile memory, which loses its content when powerto the device is switched off, and nonvolatile memory, which retains itscontent when power to the device is switched off. Like other silicontechnology, nonvolatile memory has been growing in density andperformance. This growth in density and performance has generallyfollowed Moore's law.

A further subcategory of nonvolatile memory is called flash memory.Flash memory can typically be electrically erased and reprogrammedwithout being removed and placed in a special programming device. Thegrowth of battery powered electronics such as mobile phones, digitalcameras, personal digital assistants (PDAs), and MP3 players has fueleddemand for flash memory. Flash memory may be used to store suchinformation as firmware, identification and security codes, trimming ofanalog functions, system parameters, and user programmable options.Thus, flash memory devices are now included in virtually all modernelectronic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a memory system with a controllerand a memory device.

FIG. 2 is a block diagram illustrating a memory system that includes aserial interface.

FIG. 3 is a block diagram illustrating a memory system with a controllerand multiple memory devices.

FIG. 4 is a block diagram illustrating a memory system with multiplememory devices that includes a serial interface connect in a ringtopology.

FIG. 5 is a timing diagram illustrating the operation of a memory deviceinterface.

FIG. 6 is a timing diagram illustrating a read operation communicatedvia a memory device interface.

FIG. 7 is a flow diagram illustrating a method of saving power bysuspending and reactivating an interface clock.

FIG. 8 is a flow diagram illustrating a method of saving power bysuspending and reactivating an interface clock during a read commandsequence.

FIG. 9 is a flowchart illustrating a method of operating a memory deviceinterface to save power.

FIG. 10 illustrates a block diagram of a computer system.

DETAILED DESCRIPTION

In an embodiment, a controller and a memory device are coupled via asynchronous interface to exchange commands (e.g., read, read activate,read column, program, erase, etc.) and data. The timing of the interfaceis controlled by a clock signal driven by the controller. The controllersends a command, to the memory device, via the interface. In anembodiment, the memory device samples the command synchronously withrespect to the clock signal. If there are no additional commands to besent via the interface, the controller may suspend the clock signal.When the memory device is ready (perhaps tens of microseconds later),the memory device drives a signal back to the controller. The timing ofthis signal is not dependent upon the clock signal. Receipt of thissignal by the controller indicates that the memory device is ready(e.g., the command is complete or nearly complete) and the clock signalshould be resumed so that a status of the command can be returned viathe interface, or another command issued via the interface. The memorydevice then may also use the clock signal to output the data (e.g., inthe case of the command being a read command) such that the data isoutput by the memory device synchronously with respect to the clocksignal. Stopping the clock can save power.

In another embodiment, the controller and the memory device are coupledvia two interfaces. One of these interfaces is a parallel interface usedto communicate commands (a.k.a., transactions) to and from the memorydevice. The other is a serial interface used to communicate a status(e.g., ready/busy, pass/fail) of these transactions. The serialinterface may be coupled to multiple memory devices in a daisy-chain orring-topology fashion.

The timing of the parallel and serial interfaces is controlled by theclock signal from the controller. When there are no transactions to becommunicated via the parallel interface, no status to be communicatedvia the serial interface, and the clock is not needed by the memorydevice for internal operations, the controller stops toggling the clocksignal. When there is a transaction to be communicated, a status to becommunicated, or the clock is now needed by the memory device, a signalis sent to the controller indicating the clock signal should bere-enabled. This signal may be an open-drain type signal so thatmultiple memory devices may request the controller to turn the clocksignal back on using a single signal line. In response to this signal,the controller resumes toggling the clock signal.

FIG. 1 is a block diagram illustrating a memory system with a controllerand a memory device in accordance with an embodiment. In FIG. 1 , memorysystem 100 comprises controller 110, memory device 130, and interconnect150. Controller 110 and memory device 130 are integrated circuit typedevices, such as one commonly referred to as a “chip”. A memorycontroller, such as controller 110, manages the flow of data going toand from memory devices. A memory controller can be a separate, standalone chip, or integrated into another chip. For example, a memorycontroller may be included on a single die with a microprocessor, orincluded as part of a more complex integrated circuit system such as ablock of a system on a chip (SOC).

Controller 110 includes control logic 112, interface 111, clock driver121, and resume signal receiver 122. Interface 111 includes interfacelogic 114, interface drivers 124-125, and interface receivers 127-128.Memory device 130 includes control logic 132, interface 131, memory core136, clock receiver 141, and resume signal driver 142. Interface 131includes interface logic 134, interface drivers 144-145, and interfacereceivers 147-148. Interconnect 150 includes clock signal line 151,resume signal line 152, and interface signal lines 153. Control logic112 is operatively coupled to clock signal driver 121, resume signalreceiver 122, and interface logic 114. Clock signal driver 121 iscoupled to drive a clock signal onto clock signal line 151. Resumesignal receiver 122 receives a resume signal from resume signal line152. Interface drivers 124-125 and interface receivers 127-128 arecoupled to interface logic 114. Interface drivers 124-125 and interfacereceivers 127-128 send and receive, respectively, signals carried byinterface signal lines 153.

Control logic 132 is operatively coupled to clock signal receiver 141,resume signal driver 142, interface logic 134, and memory core 136.Clock signal receiver 141 is coupled to receive a clock signal fromclock signal line 151. Clock signal receiver is coupled to send thereceived clock signal to control logic 132 and interface logic 134.Thus, the clock signal driven by controller 110 is operatively coupledto control logic 132 and interface logic 134.

Resume signal driver 142 is coupled to send a resume signal via resumesignal line 152. A resume signal generated by control logic 132 may bereceived by control logic 112 via resume signal driver 142, resumesignal line 152, and resume signal receiver 122. In response to thestate of the resume signal received by control logic 112, control logic112 may selectively enable and disable the toggling of the clock signalsent to memory device 130 (and thus control logic 132 and interfacelogic 134) via clock signal line 151.

Interface drivers 144-145 and interface receivers 147-148 are coupled tointerface logic 134. Interface logic 134 is also coupled to controllogic 132 and memory core 136. Interface drivers 144-145 and interfacereceivers 147-148 are coupled to send and receive, respectively, signalscarried by interface signal lines 153.

In an embodiment, interface 111, under the control of control logic 112,outputs commands and data to memory device 130. This data may includedata being sent to memory core 136 for storage. Likewise, under thecontrol of control logic 132, interface 131 outputs responses to thesecommands and data to controller 110. This data may include informationretrieved from memory core 136. Memory core 136 may be a nonvolatile orflash memory core.

The protocols for transferring commands and data via interfaces 111 and131 may be specified according to, or compliant with a standard. Forexample, to facilitate NAND Flash integration into consumer electronicproducts, computing platforms, and any other application that requiressolid state mass storage, the Open NAND Flash Interface industry workinggroup have promulgated several specifications that define a standardizedNAND Flash device interfaces. These specifications define standardizedcomponent-level interface specifications, connector, and module formfactor specifications for NAND Flash. These standards are available atwww.onfi.org. Likewise, a standard that details interfaces calledToggle-mode DDR NAND has been proposed to JEDEC for standardization.

Commands sent by controller 110 to memory device 130 may include thefollowing functions: read, read activate, read column, read statusregister, program, and erase. A command, or one or more indications of aresult, status, or data, or a combination of these, may be referred toas a transaction.

The read function results in memory device 130 fetching a page frommemory core 136 into on-chip page registers and returning the requestedcolumn data. The read activate function results in memory device 130fetching a page from memory core 136 into on-chip page registers. Theread column function results in memory device 130 returning therequested column data from the on-chip page registers to controller 110.The read status function results in a status being sent from memorydevice 130 to controller 110. The program function first results inmemory device 130 setting itself up to be programmed. Then, data sent tomemory device 130 by controller 110 is written to an addressed locationin memory core 136. The erase function performs an erase of an addressedsector.

Commands sent by controller 110 to memory device 130 may take differentamounts of time for control logic 132 to complete. In addition, theamount of time a particular command takes to be executed by the memorydevice 130 may vary each time that command is issued to the memorydevice. For example, data to be written into memory core 136 istypically applied to memory core 136 along with the appropriate controlsignals and programming voltages. Memory core 136 is then set to aprogram verify mode and the data just written is read back by controllogic 132. If the read data does not match the written data, the writeprocess may be repeated up to a maximum number of retries. Once the readdata matches the write data, the program function may halt. Thus, theprogram function may take an unknown number of programming cycles tocomplete.

In an embodiment, controller 110 drives a clock signal to memory device130 via clock signal line 151. Typically, this clock signal when driven,will toggle periodically at a predetermined frequency. Controller 110also outputs a command to memory device 130 via interface 111. Memorydevice 130 may use the clock signal to sample the command fromcontroller 110. After this command is output, controller 110 may ceaseto provide the clock signal. For example, if controller 110 has noadditional commands to send to memory device 130, controller 110 maystop toggling the clock signal. In another example, controller 110 maycease to provide the clock based on a status or condition associatedwith the command. For example, controller 110 may stop providing theclock signal because it has associated a condition, such as a waitperiod or wait status, with the command.

After memory device 130 has processed the command sent by controller110, or for some other reason needs the clock signal, memory device 130drives a resume signal to controller 110 via resume signal line 152.Upon receiving the resume signal, controller 110 resumes driving theclock signal to memory device 130. This allows controller 110 and memorydevice 130 to communicate via interface 111 and interface 131,respectively. This communication may be based on timing associated with(or synchronized by) the clock signal. Memory device 130 may return anindicator of status associated with the command via interface signallines 153. In an example, after controller 110 resumes driving theclock, controller 110 or memory device 130 may output additional data orcommands to interface signal lines 153. Examples of additional data thatmay be output includes portions of burst-mode or page-mode access data.

In an embodiment, memory device 130 may receive the command fromcontroller 110 asynchronously or timed by a signal other that clocksignal line 151. In this case, the resume signal driven by memory device130 may indicate that controller 110 is to provide a clock or strobesignal to time a data transfer from memory device 130 to controller 110.This strobe signal may be used instead of, or in combination with, theresumed clock signal on clock signal line 151. For example, the clocksignal on clock signal line 151 may be used to synchronize the sendingof commands by the controller and a strobe signal, sent on anothersignal line (not shown) by the controller may be used as a timingreference for the transfer of data. A strobe signal, in an embodiment,is a source synchronous signal that travels alongside the data as itpropagates between the controller and memory device and is used by thereceiver to capture the data. The start and end of the toggling of thestrobe may indicate the start and end of the data transfer. A strobesignal may also have a preamble to indicate the start of the data burst.

Memory device 130 receives the clock signal on clock signal line 151 viareceiver 141. In an embodiment, memory device 130 receives the commandsent by controller 110 synchronously with respect to the clock signal.In other words, receivers 147-148 of interface 131 may sample signalspresent on interface signal lines, based on timing associated with theclock signal received on clock signal line 151. A command received bymemory device 130 may be a read command that specifies an access to datastored by memory core 136. After sending the read command to memorydevice 130, controller 110 may stop sending the clock signal.

A delay time period may elapse while memory device 130 processes theread command and accesses the requested data from memory core 136.During this delay time period, memory device 130 may be operatedaccording to internally generated timing signals. Once a command isstarted, these internally generated timing signals are not dependentupon a received clock signal. The clock signal received via clock signalline 151 may only be necessary for synchronizing high speed transfers onthe interface(s) between controller 110 and memory device 130.

After a delay time period has elapsed, memory device 130 may drive theresume signal to controller 110. Memory device 130 may drive the resumesignal before the requested data is ready to be output. Typically,memory device 130 will be driving the clock resume signal when the clocksignal is not toggling. Once both the clock signal is resumed, and theaccessed data is ready to be output, memory device 130 outputs theaccessed data using drivers 144-145. The accessed data may be output bydrivers 144-145 based on timing associated with the transitions of theclock signal (i.e., output drivers 144-145 output the data synchronouslywith respect to the clock signal).

FIG. 2 is a block diagram illustrating a memory system that includes aserial interface. In FIG. 2 , memory system 200 comprises controller210, memory device 230, and interconnect 250. Controller 210 may be anintegrated circuit. Memory device 230 may be an integrated circuit.

Controller 210 includes control logic 212 interface 211, clock driver221, and resume signal receiver 222. Interface 211 includes interfacelogic 214, interface drivers 224-225, and interface receivers 227-228.Memory device 230 includes control logic 232, interface 231, memory core236, clock receiver 241, and resume signal driver 242. Interface 231includes interface logic 234, interface drivers 244-245, and interfacereceivers 247-248. Interconnect 250 includes clock signal line 251,resume signal line 252, and interface signal lines 253.

In FIG. 2 , controller 210, control logic 212, interface 211 (andinterface 211's components 214, 224-225, 227-228), clock driver 221, andresume signal receiver 222 are interconnected in FIG. 2 and perform muchthe same functions as described previously with respect to controller110, control logic 112, interface 111 (and interface 111's components114, 124-125, 127-128), clock driver 121, and resume signal receiver122, respectively, in FIG. 1 . Memory device 230, control logic 232,interface 231 (and interface 231's components 234, 244-245, 247-248),memory core 236, clock receiver 241, and resume signal driver 242 areinterconnected in FIG. 2 and perform much of the same functions asdescribed previously with respect to memory device 130, control logic132, interface 131 (and interface 131's components 134, 144-145,147-148), memory core 136, clock receiver 141, and resume signal driver142, respectively, in FIG. 1 . Interconnect 250, clock signal line 251,resume signal line 252, and interface signal lines 253 interconnect andcarry corresponding signals between controller 210 and memory device 230as interconnect 150, clock signal line 151, resume signal line 152, andinterface signal lines 153, respectively, in FIG. 1 .

In FIG. 2 , however, controller 210 further includes interface 213.Memory device further includes interface 233. Interconnect 250 furtherincludes interface signal lines 254. Thus, controller 210 and memorydevice 230 in FIG. 2 are connected by the additional interfaces 213 and233 not shown with respect to controller 110 and memory device 130 inFIG. 1 .

Interface 213 includes interface logic 215, serial driver 226, andserial receiver 229. Interface 233 includes interface logic 235, serialdriver 246, and serial receiver 249. Serial driver 226 may drive aserial bit stream onto one of interface signal lines 254 to be receivedby serial receiver 249. Serial driver 246 may drive a serial bit streamonto one of interface signal lines 254 to be received by serial receiver229.

Interface 213 of controller 210 and interface 233 of memory device 230form a bidirectional interface for communicating between controller 210and memory device 230. This bidirectional interface is in addition to,and may be independent from, a parallel interface formed by interfaces211 and 231. Information may be communicated in a serial manner viainterface signal lines 254 between controller 210 and memory device 230.This information may include transactions that assign an identificationnumber to memory device 230; ready/busy/done status indicatorsassociated with transactions, command, or processes; and, transactionpass/fail status indicators. In another embodiment, interfaces 213 and233 of controller 210 and memory device 230, respectively, may haveadditional signal lines in order to communicate information in aparallel manner.

Similar to controller 110, controller 210 may drive a clock signal tomemory device 230. This clock signal, when driven, typically has anominally stable, predetermined frequency. Controller 210 may sendcommands or transactions via interface 211. Controller 210 may sendcommands or transactions via interface 213. These commands ortransactions may be sampled using the clock signal. After a transactionis output, controller 210 may stop toggling the clock signal. Controller210 may stop sending the clock signal when it has no additional commandsto send. A status or condition associated with a command may also resultin controller 210 stopping the clock signal. For example, controller 210may stop the clock signal during a transaction condition, such as awaiting for a command to complete, or waiting for a response from memorydevice 230.

When memory device 230 needs the clock signal, memory device 230 maydrive a resume signal via resume signal line 252. Controller 210 resumesdriving the clock signal in response to receiving the resume signal.Once the clock signal is resumed, controller 210 and memory device 230may communicate via interfaces 211 and 231, or via interfaces 213 and233. These communications may be governed according to timing associatedwith the clock signal.

Memory device 230 may return an indicator of status associated with acommand via serial interface signal lines 254. This indicator may besent as a serial bit stream. The timing of the bits sent by memorydevice 230 may be governed by the resumed clock signal. Controller 210or memory device 230 may send additional data or commands via either ofinterface signal lines 253 or 254. Examples of additional data that maybe sent include serial data received from other memory devices (notshown), and portions of burst-mode or page-mode access data.

As discussed previously, memory device 230 may receive commands or datasynchronously with respect to the clock signal. In other words,receivers 247-248 of interface 231 and/or receiver 249 of interface 249may sample based on the timing of the clock signal. Likewise, drivers244-246 may drive signals based on the timing of the clock signal.

Commands received by memory device 230 may include read or programcommands. Read or program commands typically involve an access to memorycore 236. While controller 210 waits for one or more commands tocomplete, controller 210 may stop sending the clock signal.

Memory device 230 may run a timer while it processes one or morecommands, and accesses memory core 236. This timer may initiate anaction by memory device 230 after a delay time period elapses. Thisaction may include driving the resume signal. The delay time period maycorrespond to an anticipated completion time for a command beingprocessed. The delay time period may be set such that once the resumesignal is driven, and controller 210 responds by resuming the clocksignal, the clock signal is being received by memory device 230 beforethe command completes. In this way, memory device 230 will beimmediately ready to send a result, or data, to controller 210 viaeither of interface signal lines 253 or interface signal lines 254 whenthe command completes.

FIG. 3 is a block diagram illustrating a memory system with a controllerand multiple memory devices. Memory system 300 comprises controller 310,memory device 320, memory device 321 and memory device 322. Controller310 may correspond to controller 110 or controller 210. Any of memorydevices 320-322 may correspond to memory device 130. Controller 310 iscoupled to each of memory devices 320-322 by a sharedaddress/command/data bus 330. Address/command/data bus 330 maycorrespond to interface signal lines 153.

Controller 310 drives a clock signal via clock signal line 331 to aclock input of each of memory devices 320-322. This clock signal maycorrespond to the clock signal carried by clock signal line 151. Eachmemory device 320-322 may drive a resume signal via resume signal line332 to controller 310. This resume signal may correspond to the resumesignal carried by resume signal line 152. Since multiple memory devices320-322 may drive resume signal 332, resume signal 332 may be configuredas a “wired-OR” type signal connected in common to each memory device320-322. A wired-OR signal line typically has a single passive pull-updevice (e.g., termination resistor). Each memory device wishing toassert the line only pulling the signal line down (or to anotherpredetermined logic level). Typically, this is done with open-drain oropen-collector type output drivers. Thus, drive fights where one deviceis trying to pull a line to a high level while another device is tryingto pull the line to a low level are avoided. This also results in thelogical OR'ing of all of the outputs being asserted.

FIG. 4 is a block diagram illustrating a memory system with multiplememory devices that includes a serial interface connected in a ringtopology. Memory system 400 comprises controller 410, memory device 420,memory device 421 and memory device 422. Controller 410 may correspondto controller 210. Any of memory devices 420-422 may correspond tomemory device 230. Controller 410 is coupled to each of memory devices420-422 by a shared address/command/data bus 430. Address/command/databus 430 may correspond to interface signal lines 253.

Controller 410 drives a clock signal via clock signal line 431 to aclock input of each of memory devices 420-422. This clock signal maycorrespond to the clock signal carried by clock signal line 251. Eachmemory device 420-422 may drive a resume signal via resume signal line432 to controller 410. This resume signal may correspond to the resumesignal carried by resume signal line 252. Since multiple memory devices420-422 may drive resume signal 432, resume signal 432 may be configuredas a wired-OR type signal.

Controller 410 also interfaces to memory devices 420-422 via aring-topology serial interface. This serial interface may correspond tothe signals carried by interface signal lines 254. The serial interfaceis comprised of serial signal lines 440-443. Serial signal line 440 isdriven by a serial output (SO) of controller 410 and received by aserial input (SI) of memory device 420. Serial signal line 441 is drivenby the SO output of memory device 420 and received by the SI input ofmemory device 421. Serial signal line 442 is driven by the SO output ofmemory device 421 and received by the SI input of memory device 422.Serial signal line 443 is driven by the SO output of memory device 422and received by the SI input of controller 410. Thus, it can be seenthat the SI inputs and SO outputs of memory system 400 are connected toform a ring topology. This allows serial data to be passed from deviceto device until it reaches a destination device. This also allows any ofthe devices on the ring to originate data (or commands) and have itpassed along by the other devices until it reaches its destination.Typically, the destination device of a command or data originated by oneof memory devices 420-422 will be controller 410. Typically, commands ordata originated by controller 410 will be destined for one of memorydevices 420-422.

FIG. 5 is a timing diagram illustrating the operation of a memory deviceinterface according to an embodiment. FIG. 5 illustrates a clock signal(CK), a controller output interface (controller out), a resume signal(CR #), and a memory output interface (memory out).

At the start of the timing diagram of FIG. 5 , CK is shown toggling witha period of approximately 2.5 nS. Controller out and memory out areshown as undriven/unknown/don't care. CR #is not asserted. Later in thetiming diagram, while CK is toggling, a command (CMD) is driven oncontroller out. For example, a command may be driven by controller 110onto interface signal lines 153.

After the command is driven, CK stops toggling and assumes a steadystate at a high logic level (510). For example, controller 110 may stopsending clock signal CK. After approximately 25 μS has elapsed (e.g.,while a memory device processes the command), CR #is asserted. Forexample, CR #may be asserted by memory device 130. In response to theassertion of CR #, CK begins toggling again (511). For example,controller 110 may resume sending the clock signal CK. Once the clockhas resumed toggling, a status is driven on memory out. For example,data or status may be driven by memory device 130 onto interface signallines 153. After the status is finished being driven, CR #is de-asserted(512). For example, memory device 130 may de-assert the resume signalafter it has driven a status (and/or data) to controller 110. Controller110 may then be free to drive another command, or once again stoptoggling the clock.

FIG. 6 is a timing diagram illustrating a read operation communicatedvia a memory device interface. FIG. 6 illustrates a clock signal (CK), acontroller output interface (controller out), a resume signal (CR #), aserial interface (serial interface), and a memory output interface(memory out).

At the start of the timing diagram of FIG. 6 , CK is shown toggling witha period of approximately 2.5 nS. Controller out, serial interface, andmemory out are shown as undriven/unknown/don't care. CR #is notasserted. Later in the timing diagram, while CK is toggling, a firstread command (CMD1) is driven on controller out. The read command, inthis example, is driven by controller 210 to memory device 230.

After CMD1 is driven, CK stops toggling and assumes a steady state at ahigh logic level (610). For example, controller 210 may stop sendingclock signal CK. After approximately 25 μS has elapsed, such as whilememory device 230 processes CMD1, CR #is asserted. For example, CR #maybe asserted by memory device 230 when it is done, or almost done,processing CMD1. CK begins toggling again as a result of the assertionof CR # (611). For example, as a result of CR #being asserted,controller 210 may resume sending the clock signal CK. Once the clockhas resumed toggling, a status is driven on the serial interface. Forexample, once the clock has resumed toggling, data or status may bedriven by memory device 230 on interface signal lines 254 according totiming specified by CK. After the status is finished being driven, CR#is de-asserted (612). For example, memory device 230 may de-assert theresume signal after it has driven a status (and/or data) to controller210 via a serial interface.

In response to receiving the status via the serial interface, a secondcommand (CMD2) is driven on controller out (613). For example, if CMD1was a read activate command, a read column command may be issued bycontroller 210 to complete the transfer of data from memory device 230.After CMD2 is driven, data may be driven on memory out (614). Forexample, in response to the read column command, memory device 230 maysend data read from memory core 236 to controller 210 via a parallelinterface. Controller 210 may then be free to drive another command, oronce again stop toggling the clock.

FIG. 7 is a flow diagram illustrating a method of saving power bysuspending and reactivating an interface clock. An active (i.e.,toggling) clock signal is sent from controller 110 to memory device 130.While the active clock signal is being sent, memory device 130 sends aninactive clock resume signal to controller 110. A data access command isthen sent by controller 110 to memory device 130. For example,controller 110 may send a read or write data command to memory device130.

After the data access command is sent, controller 110 sends an inactive(i.e., non-toggling) clock signal to memory device 130. In other words,controller 110 stops sending a clock signal to memory device 130 andholds the clock signal steady at a predetermined logic level. Holdingthe clock signal steady (i.e., inactive) saves at least the amount ofpower it would take to drive a toggling clock signal. It may also savepower consumed by memory devices (e.g., 320-322) that were not thetarget of the data access command by not switching some of theirinternal circuits.

Memory device 130 then sends an active clock resume signal to controller110. In other words, memory device 130 asserts the clock resume signal.Memory device 130 may send the active clock resume signal before orafter it has completed processing the data access command. In responseto receiving the active clock resume signal, controller 110 resumessending an active clock signal to memory device 130.

FIG. 8 is a flow diagram illustrating a method of saving power bysuspending and reactivating an interface clock during a read commandsequence. An active clock signal is sent from controller 210 to memorydevice 230. Memory device 230 sends an inactive clock resume signal tocontroller 210. A first command is then sent by controller 210 to memorydevice 230. For example, controller 210 may send a read activate commandto memory device 230 via interface signal lines 253.

After the first command is sent, controller 210 sends an inactive clocksignal to memory device 230. Sending an inactive clock signal saves thepower it would take to drive a toggling clock signal. Memory device 230then sends an active clock resume signal to controller 210. As discussedpreviously, memory device 230 may send the active clock resume signalbefore or after it has completed processing the first command. Inresponse to receiving the active clock resume signal, controller 210resumes sending an active clock signal to memory device 230.

The active clock signal allows memory device 230 to send a commandstatus (or result) back to controller 210. Memory device 230 may sendthe command status via a serial bus, or via a parallel bus. The timingof signals on these busses may be controlled by the active clock signal.In response to receiving the command status from memory device 230,controller 210 sends a second command. For example, controller 210 maysend a read column command to memory device 230. In response toreceiving the second command, memory device 230 sends read data tocontroller 210. Memory device 230 may send the read data via a parallelinterface.

FIG. 9 is a flowchart illustrating a method of operating a memory deviceinterface to save power. The steps illustrated in FIG. 9 may beperformed by one or more elements of memory system 100, memory system200, memory system 300, or memory system 400.

A clock signal is sent to a memory device (902). For example, controller410 may send a clock signal to memory device 420 via clock signal line431. A first command is sent, via a first interface, to the memorydevice (904). For example, controller 410 may send a read activatecommand to memory device 420 via address/command/data bus 430. Sendingof the clock signal is halted after sending the first command (906). Forexample, controller 410 may stop sending a clock signal on clock signalline 431. After halting the sending of the clock signal, a signal isreceived, from the memory device, to resume sending of the clock signal(908). For example, controller 410 may receive, via resume signal line432, a signal to resume sending a clock signal via clock signal line431.

In response to receiving the signal to resume sending the clock signal,the sending of the clock signal is resumed (910). For example,controller 410 may resume driving a clock signal on clock signal line431 after receiving a clock resume signal on resume signal line 432. Aresult associated with the first command is received, synchronized withrespect to the resumed clock signal, from the memory device (912). Forexample, a result associated with the read activate command may bereceived by controller 410 via serial interface line 443. The serialbits associated with this command may be sent and received with timingdetermined by the resumed clock signal sent on clock signal line 431. Inanother example, a result associated with the read activate command maybe received by controller 410 via address/command/data bus 430synchronized with respect to the clock signal.

The methods, systems and devices described above may be implemented incomputer systems, or stored by computer systems. The methods describedabove may also be stored on a computer readable medium. Devices,circuits, and systems described herein may be implemented usingcomputer-aided design tools available in the art, and embodied bycomputer-readable files containing software descriptions of suchcircuits. This includes, but is not limited to memory systems 100, 200,300, and 400 and their components. These software descriptions may be:behavioral, register transfer, logic component, transistor and layoutgeometry-level descriptions. Moreover, the software descriptions may bestored on storage media or communicated by carrier waves.

Data formats in which such descriptions may be implemented include, butare not limited to: formats supporting behavioral languages like C,formats supporting register transfer level (RTL) languages like Verilogand VHDL, formats supporting geometry description languages (such asGDSII, GDSIII, GDSIV, CIF, and MEBES), and other suitable formats andlanguages. Moreover, data transfers of such files on machine-readablemedia may be done electronically over the diverse media on the Internetor, for example, via email. Note that physical files may be implementedon machine-readable media such as: 4 mm magnetic tape, 8 mm magnetictape, 3½ inch floppy media, CDs, DVDs, and so on.

FIG. 10 illustrates a block diagram of a computer system. Computersystem 1000 includes communication interface 1020, processing system1030, storage system 1040, and user interface 1060. Processing system1030 is operatively coupled to storage system 1040. Storage system 1040stores software 1050 and data 1070. Storage system 1040 may include oneor more of memory systems 100, 200, 300, or 400. Processing system 1030is operatively coupled to communication interface 1020 and userinterface 1060. Computer system 1000 may comprise a programmedgeneral-purpose computer. Computer system 1000 may include amicroprocessor. Computer system 1000 may comprise programmable orspecial purpose circuitry. Computer system 1000 may be distributed amongmultiple devices, processors, storage, and/or interfaces that togethercomprise elements 1020-1070.

Communication interface 1020 may comprise a network interface, modem,port, bus, link, transceiver, or other communication device.Communication interface 1020 may be distributed among multiplecommunication devices. Processing system 1030 may comprise amicroprocessor, microcontroller, logic circuit, or other processingdevice. Processing system 1030 may be distributed among multipleprocessing devices. User interface 1060 may comprise a keyboard, mouse,voice recognition interface, microphone and speakers, graphical display,touch screen, or other type of user interface device. User interface1060 may be distributed among multiple interface devices. Storage system1040 may comprise a disk, tape, integrated circuit, RAM, ROM, EEPROM,flash memory, network storage, server, or other memory function. Storagesystem 1040 may include computer readable medium. Storage system 1040may be distributed among multiple memory devices.

Processing system 1030 retrieves and executes software 1050 from storagesystem 1040. Processing system may retrieve and store data 1070.Processing system may also retrieve and store data via communicationinterface 1020. Processing system 1050 may create or modify software1050 or data 1070 to achieve a tangible result. Processing system maycontrol communication interface 1020 or user interface 1070 to achieve atangible result. Processing system may retrieve and execute remotelystored software via communication interface 1020.

Software 1050 and remotely stored software may comprise an operatingsystem, utilities, drivers, networking software, and other softwaretypically executed by a computer system. Software 1050 may comprise anapplication program, applet, firmware, or other form of machine-readableprocessing instructions typically executed by a computer system. Whenexecuted by processing system 1030, software 1050 or remotely storedsoftware may direct computer system 1000 to operate as described herein.

The above description and associated figures teach the best mode of theinvention. The following claims specify the scope of the invention. Notethat some aspects of the best mode may not fall within the scope of theinvention as specified by the claims. Those skilled in the art willappreciate that the features described above can be combined in variousways to form multiple variations of the invention. As a result, theinvention is not limited to the specific embodiments described above,but only by the following claims and their equivalents.

What is claimed is:
 1. A memory device, comprising: an interface toreceive a first command, timed by a clock signal, that specifies a readof a status register; a first receiver circuit to receive the clocksignal; a first driver circuit to serially transmit information from thestatus register to an external device; and a second driver circuit todrive a resume signal that indicates whether the memory device is readyto receive the clock signal.
 2. The memory device of claim 1, whereinthe clock signal is deactivated based at least in part on the resumesignal.
 3. The memory device of claim 1, wherein the first commandcomprises a plurality of bits that are received synchronously withrespect to the clock signal.
 4. The memory device of claim 1, whereinthe second driver circuit is configured for wired-OR operation of theresume signal with at least one additional memory device.
 5. The memorydevice of claim 1, wherein the interface is to receive a second commandthat specifies a read of a memory array.
 6. The memory device of claim5, further comprising: a plurality of drivers to transmit data from theread of the memory array.
 7. The memory device of claim 6, wherein theplurality of drivers transmit the data from the read of the memory arraysynchronously with respect to the clock signal.
 8. A memory device,comprising: a status register; an interface to receive a first commandthat indicates a read of the status register; a first driver circuit to,in response to at least the first command, serially transmit informationfrom the status register to an external device; and a second drivercircuit to drive a resume signal that indicates whether the memorydevice is ready to receive a clock signal that synchronizes the firstdriver circuit.
 9. The memory device of claim 8, wherein the interfaceis to receive a second command that indicates a read of memory arraydata.
 10. The memory device of claim 9, further comprising: a pluralityof drivers to transmit the memory array data.
 11. The memory device ofclaim 10, wherein the plurality of drivers transmit the memory arraydata synchronously with respect to the clock signal.
 12. The memorydevice of claim 11, wherein the clock signal is to be deactivated by anexternal device based at least in part on the resume signal.
 13. Thememory device of claim 12, wherein the first command comprises aplurality of bits that are received synchronously with respect to theclock signal.
 14. The memory device of claim 13, wherein the seconddriver circuit is configured for wired-OR operation of the resume signalwith at least one additional memory device.
 15. A method of operating amemory device, comprising: receiving, by an interface and timed by aselectively activated clock signal, a first command, the first commandindicating a read of a status register; receiving, by first receivercircuit, the selectively activated clock signal; serially transmitting,based at least in part on the first command, by a first driver circuit,and to an external device, information from the status register; andtransmitting, by a second driver circuit, a resume signal that indicateswhether the memory device is ready to receive the selectively activatedclock signal.
 16. The method of claim 15, wherein the selectivelyactivated clock signal is deactivated based at least in part on theresume signal.
 17. The method of claim 16, wherein the first commandcomprises a plurality of bits that are received synchronously withrespect to the selectively activated clock signal.
 18. The method ofclaim 17, wherein the second driver circuit is configured for wired-ORoperation of the resume signal with at least one additional memorydevice.
 19. The method of claim 18, wherein the interface is to receivea second command that specifies a read of memory array data.
 20. Themethod of claim 19, further comprising: transmitting, using a pluralityof drivers and timed by the selectively activated clock signal, thememory array data.